if you get the 19.99 plan, you can use either the wap or the internet2 apn - all the 19.99 plan does is open up the ports so that you can use tmo's network without the use of a proxy
for different regions, different apn's work faster - for me, the wap.voicestream.com apn is faster than the internet2 apn (so i use the wap apn with my total internet plan)
right now - there IS some sort of work around for the 5.99 plan although i dont have too many details - all it does is allow you to use proxy settings with the iphone
heres the thread over at hackint0sh that discusses it:
http://www.hackint0sh.org/forum/showthread.php?t=6365
i havent tried this so ymmv - i upgraded to the 19.99 plan because i could make use of the hotspots (i figure, you cant beat unlimited data + free wifi hotspots at that price)